Cryptology was, of course, essential in those days, because there was no secure form of long distance communication. Unless you could afford your own couriers to send single messages (who could still be captured or otherwise waylaid) letters were carried by traders and other travelers who were going in the direction of your addressee. These would be exchanged at various points, such as inns and taverns in order to route them to their destinations.

All of the founders had a keen interest in codes and ciphers, since they never would have assumed that any correspondence was physically secure from being read by any number of people along the way.

Smith v. Maryland, 442 U.S. 735 (1979),[1] was a case in which the Supreme Court of the United States held that the installation and use of the pen register was not a "search" within the meaning of the Fourth Amendment, and hence no warrant was required. The pen register was installed on telephone company property at the telephone company's central offices. In the Majority opinion, Justice Blackmun rejected the idea that the installation and use of a pen registry constitutes a violation of the "legitimate expectation of privacy" since the numbers would be available to and recorded by the phone company anyway.
Background

In Katz v. United States (1967), the United States Supreme Court established its "reasonable expectation of privacy" test. It overturned Olmstead v. United States and held that wiretaps were unconstitutional searches, because there was a reasonable expectation that the communication would be private. The government was then required to get a warrant to execute a wiretap.

In Smith v. Maryland, the Supreme Court held that a pen register is not a search because the "petitioner voluntarily conveyed numerical information to the telephone company." Since the defendant had disclosed the dialed numbers to the telephone company so they could connect his call, he did not have a reasonable expectation of privacy in the numbers he dialed. The court did not distinguish between disclosing the numbers to a human operator or just the automatic equipment used by the telephone company.

The NSA program reaches into homes and businesses across the nation by amassing information about the calls of ordinary Americans — most of whom aren't suspected of any crime. This program does not involve the NSA listening to or recording conversations. But the spy agency is using the data to analyze calling patterns in an effort to detect terrorist activity, sources said in separate interviews.